They're stuck like this, silently.
"I'm working on it. I'll figure it out." Shisui tells him, poring over the scrolls they stole. They are alone in Shisui's apartment, Itachi learning family history, and he feels sickened by them all, and he doesn't want the Mangekyou – doesn't need it yet.
What Itachi wants more is for Shisui to let him stay the night, but that's impossible. His father would get curious if he refused to go back.
Itachi can wait, but it is he, not Shisui who is stuck alone with his father at home. He could say so, but he suspects Shisui already knows anyway.
Yeah they've both got their own secrets and can't seem to share. Shisui's just not sure Itachi will understand (he really does think this kid is just lonely and gullible) and Itachi gathers the idea that Shisui is heartless. Which is maybe not precisely true, but it's close so I guess he can be forgiven, and he's been one to err on the side of caution.
And yeah, he doesn't need the Mangekyou – his own strength hasn't finished yet, no need to jump the gun to blindness. As to the price of it (Sasuke, Shisui) I don't think they've figured that part out yet.